Structural Performance performance issues relative to extreme events
T
here have been a number of significant flooding events in recent years, ranging from the Nashville flood in May 2010 to Hurricane Irene in August 2011. Floods are one of the leading natural disasters in the United States. Average annual U.S. flood losses for each of the past 10 years (2001-2010) exceeded $2.7 billion (National Flood Insurance Program). Fortunately, engineered wood products can be considered relatively durable when temporarily exposed to floodwaters. Many of the losses involving engineered wood products can be reduced if proper measures are taken.
Dry It Out If a building or home is flooded, it is imperative to get the structure dried out as soon as possible. This is necessary to prevent mold growth and fungal decay – the latter can lead to permanent strength loss (Kirby, Wiggins). After floodwaters have receded, any standing water in a basement or crawl space should be removed. Any insulation, gypsum board, carpet and padding, or other interior finish materials that are wet should be removed. Ceramic tile floors should also be removed, as concrete topping or backer boards used underneath ceramic tiles often retain moisture. This will speed up the drying process and allow for visual inspection of the structure. Fans and dehumidifiers should be used when possible to circulate air. As stated by APA in a publication on assessing flood damage, “Depending on conditions, the drying process can take from a week or two to several months.” While the building is being dried out, temporary shoring of any wood products may be necessary to prevent permanent set, especially for primary support members that are heavily loaded. Engineered wood products should be dried out to a moisture content less than 16%, returning it to the assumed “dry use” conditions, as specified in Section 8.1.4 of the 2005 National Design Specification® (NDS®) for Wood Construction. While not a structural concern, any mold on wood should be cleaned by a detergent and water solution, as recommended by the EPA, or a 1 cup bleach per 1 gallon water solution as recommended by the CDC. The CDC also recommends that large mold infestations should be addressed by a professional who has experience with cleaning mold in buildings and homes. A moisture meter will be necessary to determine if wood members are properly dried. A handheld electrical resistance meter with pins is the most common type of moisture meter used in the field. For engineered wood products, the pins of the meter should be put in the wide face
Engineered Wood Products Exposed to Floodwaters By Adam Pittman, P.E.
Adam Pittman, P.E. is an engineer with the engineered wood products division of Weyerhaeuser. He may be reached at adam.pittman@weyerhaeuser.com.
The online version of this article contains references. Please visit www.STRUCTUREmag.org.
26 April 2012
Moisture meter with pins inserted into bottom of I-joist.
of the beam or panel. They should be inserted parallel to the grain. It should be noted that the resins in engineered wood products affect the electrical resistance and often cause readings to be higher than the actual moisture content. Rather than attempting to account for this potential difference, it is recommended that the moisture meter output be used directly with knowledge that it may be slightly conservative.
Adhesives in Engineered Wood Products Engineered wood products are manufactured by taking a tree apart, removing inconsistencies inherent in the lumber, and putting its fibers back together to take full advantage of its natural strengths. This results in high-quality products that have higher allowable design properties, and more dimensional stability than sawn lumber. Examples of engineered wood products include I-joists, structural composite lumber, oriented strand board (OSB), and plywood. Structural composite lumber (SCL) products include laminated veneer lumber (LVL), parallel strand lumber (PSL), and laminated strand lumber (LSL). One of the most common concerns about engineered wood products exposed to extreme moisture conditions such as flooding is whether the adhesive bond will be compromised, leading to delamination of individual wood veneers or strands. Engineered wood products use adhesives that are rated for exterior use, although this use category is intended for temporary moisture exposure during construction, not long-term exposure. Structural composite lumber and I-joists use adhesives adhering to ASTM D 2559 requirements while plywood and OSB meet the Exposure 1 classification required in the U.S. Department of Commerce PS-1 and PS-2 standards, respectively. The most common adhesives used to manufacture engineered wood products are phenol formaldehyde (PF), phenol resorcinol formaldehyde (PRF) and diphenylmethane diisocyanate (MDI). During the manufacturing process, bonding is caused by a chemical reaction in the adhesive. In that reaction, the adhesive becomes
